Using serial port to control modem from a guest

Is there a way for my to use serial port on a guest linux OS?

You can use a RAW parameter in the template or set it as default. You have an example on how to add a serial console in the documentation:

RAW = [ type = "kvm",
        data = "<devices><serial type=\"pty\"><source path=\"/dev/pts/5\"/><target port=\"0\"/></serial><console type=\"pty\" tty=\"/dev/pts/5\"><source path=\"/dev/pts/5\"/><target port=\"0\"/></console></devices>" ]

For more information check libvirt documentation page.

Thank You!!